Ignition material for an igniter

ABSTRACT

An electrically actuatable igniter ( 24 ) comprises a pair of electrodes ( 40 ) and ( 42 ). A heating element ( 44 ) is electrically connected between said electrodes ( 40 ) and ( 42 ). An ignition material ( 48 ) is in contact with the heating element ( 44 ). The ignition material ( 48 ) comprises a metal powder and an oxidizer that exothermically reacts with the metal powder. The metal powder includes macro-agglomerates of metal particles. The metal particles have an average diameter less than about 0.1 μm and have an oxide layer that prevents contact of the particles with the oxidizer. The ignition material ( 48 ) deflagrates when the heating element is heated to a temperature of at least about 250° C.

FIELD OF THE INVENTION

This application is a continuation-in-part of application Ser. No.09/634,384 filed Aug. 9, 2000.

The present invention relates to an igniter, and particularly relates toan ignition material for an igniter for inflating an inflatable vehicleoccupant protection device.

BACKGROUND OF THE INVENTION

An inflatable vehicle occupant protection device, such as an air bag, isinflated by inflation gas provided by an inflator. The inflatortypically contains ignitable gas generating material. The inflatorfurther includes an igniter to ignite the gas generating material.

The igniter contains a charge of ignition material. The igniter alsocontains a bridgewire that is supported in a heat transferringrelationship with the ignition material. When the igniter is actuated,an actuating level of electric current is directed through thebridgewire in the igniter. This causes the bridgewire to becomeresistively heated sufficiently to ignite the ignition material. Theignition material then produces ignition products that, in turn, ignitethe gas generating material.

SUMMARY OF THE INVENTION

The present invention is an electrically actuatable igniter. Theelectrically actuatable igniter comprises a pair of electrodes. Aheating element is electrically connected between said electrodes. Anignition material is in contact with the heating element. The ignitionmaterial comprises a metal powder and an oxidizer that exothermicallyreacts with the metal powder. The metal powder includesmacro-agglomerates of metal particles. The metal particles have anaverage diameter less than about 0.1 μm and have an oxide layer thatprevents contact of the particles with the oxidizer. The ignitionmaterial deflagrates when the heating element is heated to a temperatureof at least about 250° C.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the invention will become moreapparent to one skilled in the art upon consideration of the followingdescription of the invention and the accompanying drawings, in which:

FIG. 1 is a schematic view of a vehicle occupant protection apparatusembodying the present invention; and

FIG. 2 is an enlarged sectional view of a part of the apparatus of FIG.1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, an apparatus 10 embodying the present inventionincludes an inflator 14 and an inflatable vehicle occupant protectiondevice 26. The inflator 14 contains a gas generating material 16. Thegas generating material 16 is ignited by an igniter 24 operativelyassociated with the gas generating material 16. Electric leads 20 and 22convey electric current to and from the igniter 24. The electric currentis conveyed to the igniter 24 through a crash sensor 18 from a powersource (not shown). The crash sensor 18 is responsive to vehicledeceleration indicative of a collision. A gas flow means 28, such as anopening in the inflator 14, conveys gas, which is generated bycombustion of the gas generating material 16, to the vehicle occupantprotection device 26.

A preferred vehicle occupant protection device 26 is an air bag that isinflatable to help protect a vehicle occupant in the event of acollision. Other vehicle occupant protection devices that can be usedwith the present invention are inflatable seat belts, inflatable kneebolsters, inflatable air bags to operate knee bolsters, inflatable headliners, and inflatable side curtains.

Referring to FIG. 2, the igniter 24 has a central axis 39 and a pair ofaxially projecting electrodes 40 and 42. A heating element in the formof a bridgewire 44 is electrically connected between the electrodes 40and 42 within the igniter 24. An ignition material 48 is containedwithin the igniter 24. The ignition material surrounds and is in contactwith the bridgewire 44 so that the ignition material is in a heatreceiving relationship with the bridgewire 44.

The igniter 24 further includes a header 50, a charge cup 52 and acasing 54. The header 50 is a metal part, preferably made of 304L steel,with a generally cylindrical body 60 and a circular flange 62 projectingradially outward from one end of the body 60. A cylindrical outersurface 64 of the body 60 has a recessed portion 66 defining acircumferentially extending groove 68.

The charge cup 52 also is a metal part, and has a cylindrical side wall70 received in a tight fit over the body 60 of the header 50. The sidewall 70 of the charge cup 52 is fixed and sealed to the body 60 of theheader 50 by a circumferentially extending weld 72. The charge cup 52 isfurther secured to the header 50 by a plurality of circumferentiallyspaced indented portions 74 of the side wall 70 that are crimpedradially inward into the groove 68. In this arrangement, the side wall70 and a circular end wall 76 of the charge cup 52 together contain andhold the ignition material 48 in a heat transferring relationship withthe bridgewire 44. A plurality of thinned portions (not shown) of theend wall 76 function as stress risers that rupture under the influenceof the combustion products generated by the ignition material 48.

The casing 54 is a sleeve-shaped plastic part that is shrink-fitted ontothe header 50 and the charge cup 52 so as to insulate and partiallyencapsulate those parts. An opening 79 in the casing 54 allows ignitionproducts escaping through the ruptured thinned portions of the chargecup 52 to exit the igniter 24.

The header 50 has a pair of cylindrical inner surfaces 80 and 82 thatare axially aligned and together define a central passage 84 extendingfully through the header 50. The first electrode 40 has an inner endportion 86 extending along the entire length of the central passage 84.A pair of axially spaced apart glass seals 88 and 90 surround the firstelectrode 40 in the central passage 84, and electrically insulate thefirst electrode 40 from the header 50 and from the electrode 42.Preferably, the glass seals 88 and 90 are formed from a barium alkalisilicate glass. The electrode 42, at one end 43, seats against theheader 50 in direct contact with the header 50.

The bridgewire 44 extends from a radially extending surface 41 of thefirst electrode 40 to a radially extending surface 51 of the header 50.The bridgewire 44 has flattened opposite end portions 100 and 102, whichare fixed to the electrode surface 41 and the header surface 51 byelectrical resistance welds 104 and 106, respectively. The opposite endportions 100 and 102 of the bridgewire 44 become flattened under thepressure applied by the welding electrodes (not shown) that are used toform the resistance welds 104 and 106. The bridgewire 44 thus has anunflattened major portion 108 extending between the opposite endportions 100 and 102. The major portion 108 of the bridgewire 44 is bentso that the major portion 108 lies in a plane spaced from the plane ofthe opposite end portions 100 and 102 and from a radially extendingsurface 89 of the first glass seal 88 and the header surface 51.

The bridgewire 44, in one embodiment, is formed from a high resistancemetal alloy. A preferred metal alloy is “NICHROME”, a nickel-chromiumalloy. Other suitable alloys for forming a high resistance bridgewire 44include platinum-tungsten and 304L steel. An electrical current flow inthe bridgewire 44 resistively heats the bridgewire to temperature ofabout 250° C. to about 400° C. The heat generated by the bridge wire 44is sufficient to ignite the ignition material 48.

A semi-conductor bridge (SCB) may be used in place of the bridgewire 44.A semi-conductor bridge consists of dissimilar conductive materials suchas a thick resistive film on a ceramic substrate, a thin resistive filmdeposited on a ceramic substrate, or a semi-conductor junction diffusiondoped onto a silicon substrate. A current flow in the semi-conductorbridge heats the semi-conductor bridge to a temperature of about 250° C.to about 400° C., which is sufficient to ignite the ignition material48. Examples of semi-conductor bridges include: a substrate that isformed of ceramic material such as dense alumina (Al₂O₃), beryllia(BeO), or steatite and an alloy such as nickel-chrome,phosphorous-chrome, or tantalum nitride on the substrate.

In accordance with the present invention, the ignition material 48 is apyrotechnic composition that deflagrates when the bridgewire 44 isheated to a temperature of at least about 250° C. By deflagrate, it ismeant that the ignition material 48 undergoes an exothermic chemicalreaction producing a vigorous evolution of heat and sparks or flame thatmove through the ignition material 48 at a speed less than the speed ofsound.

The ignition material 48 of the present invention comprises a fuel andan oxidizer. The fuel is a metal powder that exothermically reacts withthe oxidizer upon actuation of the igniter 24. The metal powder of thepresent invention is produced by electro-explosion of a metal wire undercontrolled atmospheric condition. The electro-explosion of metal wire toproduce a powdered metal is well known in the art. In the process, ametal wire is placed in an inert atmosphere and connected in anelectrical circuit that includes a power source. The wire is pulsed withan electrical current sufficient to increase the temperature of themetal wire to a temperature of about 10,000° C. to about 20,000° C. At atemperature of about 10,000° C. to about 20,000° C., the metal wirevaporizes and forms metal plasma. The pulse of electric current, whichvaporizes the metal wire, also produces an electromagnetic field thatinitially contains the metal plasma. The vapor pressure of the metalplasma overcomes the electromagnetic field, and the metal plasmaexplodes into an aerosol of metal particles.

The metal particles so formed by explosion of the metal wire have anessentially spherical configuration and have an average particle sizeless than about 100 nm. Preferably, the metal particles have an averageparticle size from about 20 nm to about 100 nm. Other methods forobtaining metal particles in this size range are also known.

The metal particles agglomerate into macro-agglomerates having theconsistency of a metal powder. The macro-agglomerates have an averagediameter of about 1 μm to about 2 μ. Preferably, the macro-agglomerateshave an average diameter of about 1 μm.

Metal powders in the size range formed by the electro-explosion of ametal wire react more readily with the oxidizer of the present inventionthan metal powders formed by conventional methods such as gasatomization. The rate of reaction of the metal powder with the oxidizeris increased for metal powders formed by electro-explosion of metalwires because the activation barrier for particle reaction are reduced.One component of this is facilitated heat transfer owing to greatersurface area than metal powders formed by conventional methods. Metalpowders formed by electro-explosion have a surface area of about 15square meters per gram, which is several orders of magnitude greaterthan metal powders formed by conventional methods. Reactivity, reflectedin autoignition temperature or time to ignition involve melting thealuminum, occurs at lower average temperature where the activationbarrier is reduced. Heat release from oxidation of the metal now occursin or near the reaction front affording the significantly fasterreaction rate. It is contemplated that mixtures of electro-explodedmetal particles, electro-exploded alloys of metals and metals producedby conventional means can be used in varying proportions to modify theproperties or reactivity of the metal and its oxide surface layer. Forexample, a composition employing a mixture of 20% electro-explodedaluminum and 80% Valimet H-5 aluminum powder could be used to achievereactivity and other properties intermediate to compositions preparedwith either metal alone. Moreover, it is believed that metal powdersformed by electro-explosion have a strained crystal structure. Thisstrained crystal structure, during reaction of the metal powder with theoxidizer, undergoes exothermic rearrangement. The exothermicrearrangement of the crystal structure generates heat, which in turnfacilitate reaction of metal powder formed by electro-explosion and theoxidizer.

Preferred metal powders formed by electro-explosion are electro-explodedaluminum powder, electro-exploded titanium powder, electro-explodedcopper powder, electro-exploded zinc powder, and electro-explodedyttrium powder.

These electro-exploded metal powders are commercially available fromArgonide Co. Other manufacturers have developed processes capable ofproducing material of these and other metals of similar dimensions whichpossess the same advantages as noted for electro-exploded metal and aresuitable for use in the present invention.

These electro-exploded metal powders are preferred because, uponreaction with the oxidizer of the present invention, they form anon-toxic and environmentally benign ignition product. Moreover, theseelectro-exploded metal powders, when combined with the oxidizer ofpresent invention, form ignition materials that do not thermallydecompose at temperatures up to about 120° and do not deflagrate whenexposed to external stress such as impact.

The metal particles that form the electro-exploded aluminum powder,electro-exploded titanium powder, electro-exploded copper powder,electro-exploded zinc powder, and electro-exploded yttrium powder arenaturally coated, upon exposure to air, with a thin metal oxide layerof, respectively, aluminum oxide, titanium oxide, copper oxide, zincoxide, and yttrium oxide. The coating of metal oxide is about 1 nm toabout 30 nm thick. The coating of metal oxide passivates the metalsurface since it does not readily react with the oxidizer of the presentinvention. Alternate methods of passivating the surface are known, suchas organic or inorganic coatings. As a result, the passivating coatingacts as a buffer to prevent the metal particles from contacting andreacting with oxidizer during processing of the ignition material andstorage of the ignition material. Energy output is reduced to the extentthat a quantity of the metal is consumed during formation of thenon-reactive oxide. The preferred thickness is the minimum amount toafford safe processing, thus maximizing the reactive metal content atthe core of the particle. Thus, ignition materials comprisingelectro-exploded aluminum powder, electro-exploded titanium powder,electro-exploded copper powder, electro-exploded zinc powder, andelectro-exploded yttrium powder may be processed using conventionalprocessing techniques.

A more preferred metal powder is electro-exploded aluminum powder.Electro-exploded aluminum powder comprises macro-agglomerates ofaluminum particles. The aluminum particles have an average particle sizeof about 20 nm to about 100 nm. The macro-agglomerates have an averagediameter of about 1 μm.

The amount of metal powder in the ignition material is that amountnecessary to achieve sustained, rapid deflagration of the ignitionmaterial upon ignition. Preferably, the amount of fuel is from about 15%to about 75% by weight of the ignition material. Metal from theparticles in excess of that needed to react with the solid oxidizer isavailable to transfer heat or burn when in contact with other condensedor gaseous components of the system. This affords an additional means oftuning the performance of the system. More preferably, the amount ofmetal powder is from about 30% to about 60% by weight of the ignitionmaterial.

The oxidizer of the present invention may be any oxidizing material thatreadily reacts with the metal powder of the present invention andproduces an ignition product that is non-toxic and environmentallybenign. A preferred oxidizer is an inorganic salt oxidizer. Examples ofinorganic salt oxidizers that can be used in an ignition material of thepresent invention are alkali metal nitrates such as sodium nitrate andpotassium nitrate, alkaline earth metal nitrates such as strontiumnitrate and barium nitrate, alkali metal perchlorates such as sodiumperchlorate, potassium perchlorate, and lithium perchlorate, alkalineearth metal perchlorates, alkali metal permanganates such as potassiumpermanganate, alkali metal chlorates such as sodium chlorate, lithiumchlorate and potassium chlorate, alkaline earth metal chlorates such asmagnesium chlorate and calcium chlorate, ammonium perchlorate, ammoniumnitrate, and mixtures thereof.

Other oxidizers that may be used in the present invention are metaloxides, peroxides, and superoxides such as ferric oxide (Fe₂O₃), cupricoxide (CuO), manganese dioxide (MnO₂), and molybdenum trioxide (MoO₃).

The oxidizer is incorporated into the ignition material in the form ofparticles. The sensitivity of the ignition material to thermaldecomposition and external stress such as impact, as well as the burningrate of the ignition material, are dependent on the average particlesize of the oxidizer. If the particle size of the oxidizer incorporatedin the ignition material is less than about 0.1 μm, the ignitionmaterial can auto-ignite at temperatures below about 250° C. or uponexposure to external stress such as shock. If the average particle sizeof the oxidizer incorporated in the ignition material is greater thanabout 100 μm, the burn rate of the ignition material will not beeffective to ignite the gas generating material and actuate the vehicleoccupant protection apparatus. Preferably, the oxidizer incorporated inthe ignition material has an average particle size of about 1 μm toabout 30 μm.

The amount of oxidizer in the ignition material is that amount necessaryto achieve sustained, rapid deflagration of the ignition material uponignition. Preferably, the amount of oxidizer in the ignition material isabout 25% to about 85% by weight of the ignition material. Morepreferably, the amount of oxidizer in the ignition material is about 50%to about 75% by weight of the ignition material.

In a preferred embodiment of the present invention, the ignitionmaterial is prepared by adding the metal powder and the particulateoxidizer to a conventional mixing device, without the addition of anyprocessing aids such as solvents or binders. The metal powder andparticulate oxidizer are then mixed until the metal powder andparticulate oxidizer are uniformly dispersed. The ignition material soformed is pressed into the ignition cup 52 of the igniter 24.

Alternatively, ignition material can be prepared by admixing the metalpowder and the particulate oxidizer with a binder. Preferably, thebinder is an oxidizable organic material. Examples of oxidizable organicmaterials are organic polymers such as cellulose esters, celluloseethers, vinyl polymers, acrylates, and methacryalates, phenolaldehydes,polyamides, natural and synthetic rubber, natural resins and halogenatedpolymers.

The amount of binder mixed with the powdered metal and particulateoxidizer is that amount sufficient to form a homogenous mixture with themetal powder and particulate oxidizer without impairing the sensitivityof the ignition material to ignition by the heating element. Preferably,the amount of binder mixed the powdered metal and the particulateoxidizer is from about 1% to about 5% by weight of the ignitionmaterial.

The powdered metal, particulate oxidizer, and binder are mixed by aconventional mixture until a homogeneous mixture is formed. Thehomogeneous mixture of powdered metal, particulate oxidizer, and binderis pressed into the ignition cup 52 and allowed to dry.

When the igniter 24 is actuated, an actuating level of electric currentis directed through the bridgewire 44 between the electrodes 40 and 42.As the actuating level of the electric current is conducted through thebridgewire 44, the bridgewire 44 is heated to a temperature betweenabout 250° C. and about 400° C. The heat is transferred directly to theignition material 48. The particles of ignition material adjacent to thebridgewire 44 ignite, resulting in deflagration of the ignitionmaterial. Deflagration of the ignition material produces ignitionproducts, including heat, hot gases and hot particles at a temperatureof about 3000° C. to about 6000° C. The ignition products are spewedoutward from the igniter 24 and ignite the gas generating material.

EXAMPLE

This Example illustrates preparation of an ignition droplet inaccordance with the present invention.

25 mg of electro-exploded aluminum powder and 75 mg of particulatepotassium perchlorate are added to a mixing device (“POWERGEN” No. 35manufactured by Powergen Inc.). The electro-exploded aluminum powder iscommercially available from Argonide Co. under the trade name ALEX. Theelectro-exploded aluminum powder comprises macro-agglomerates ofaluminum particles. The aluminum particles have an average diameter ofabout 50 nm. The macro-agglomerates have an average diameter of about 1μm. The particles of the potassium perchlorate have an average diameterof about 5 microns.

The electro-exploded aluminum powder and potassium perchlorate areblended until the electro-exploded aluminum powder is uniformlydispersed with the particles of potassium perchlorate.

The ignition material so formed does not thermally decompose attemperatures up to about 120° C. and is resistant ignition by impact.The ignition material produces an ignition product upon deflagrationthat has a temperature greater than about 3000° C.

From the above description of the invention, those skilled in the artwill perceive improvements, changes and modifications. Suchimprovements, changes and modifications within the skill of the art areintended to be covered by the appended claims.

1. An electrically actuatable igniter comprising: a pair of electrodes;a heating element electrically connected between said electrodes; and anignition material in contact with said heating element, said ignitionmaterial comprising a metal powder and an oxidizer that exothermicallyreacts with said metal powder, said metal powder includingmacro-agglomerates of metal particles, said metal particles having anaverage diameter less than about 0.1 μm and having an oxide layer thatprevents contact of said particles with said oxidizer, wherein saidignition material deflagrates when the heating element is heated to atemperature of at least about 250° C.
 2. The electrically actuatableigniter of claim 1 wherein the macro-agglomerates have an averagediameter of about 1 μm to about 2 μm.
 3. The electrically actuatableigniter of claim 1 wherein said oxidizer is selected from the groupconsisting of alkali metal nitrates, alkaline earth metal nitrates,alkali metal perchlorates, alkaline earth metal perchlorates, alkalimetal chlorates, alkaline earth metal chlorates, ammonium perchlorates,ammonium nitrate, and mixtures thereof.
 4. The electrically actuatableigniter of claim 3 wherein the oxidizer has an average particle size ofabout 1 μm to about 30 μm.
 5. The electrically actuatable igniter ofclaim 1 wherein the metal powder is selected from the group consistingof electro-exploded aluminum powder, electro-exploded titanium powder,electro-exploded copper powder, electro-exploded zinc powder, andelectro-exploded yttrium powder.
 6. The electrically actuatable igniterof claim 5 wherein the electro-exploded metal powder is electro-explodedaluminum.
 7. The electrically actuatable igniter of claim 1 wherein theelectro-exploded metal powder is about 15% to about 75% by weight of theignition material.
 8. The electrically actuatable igniter of claim 1wherein the amount of oxidizer is about 25% to about 85% by weight ofthe ignition material.
 9. The electrically actuatable igniter of claim 1wherein the ignition material upon ignition deflagration produces anignition product with a temperature of about 3000° C. to about 6000° C.10. The electrically actuatable igniter of claim 1 wherein the ignitionmaterial does not thermally decompose at temperatures up to about 120°C.
 11. An electrically actuatable igniter comprising: a pair ofelectrodes; a heating element electrically connected between saidelectrodes; and an ignition material in contact with said heatingelement, said ignition material comprising an electro-exploded metalpowder and a particulate oxidizer, wherein said ignition materialdeflagrates when the heating element is heated to a temperature of atleast about 250° C.
 12. The electrically actuatable igniter of claim 11wherein said oxidizer is selected from the group consisting of alkalimetal nitrates, alkaline earth metal nitrates, alkali metalperchlorates, alkaline earth metal perchlorates, alkali metal chlorates,alkaline earth metal chlorates, ammonium perchlorate, ammonium nitrate,and mixtures thereof.
 13. The electrically actuatable igniter of claim12 wherein the oxidizer has an average particle size of about 1 μm toabout 30 μm.
 14. An electrically actuatable igniter comprising: a pairof electrodes; a heating element electrically connected between saidelectrodes; and an ignition material in contact with said heatingelement, said ignition material comprising a uniformly dispersed mixtureof a metal powder and a particulate inorganic salt oxidizer thatexothermically reacts with said metal powder, said oxidizer having anaverage particle size of about 1 μm to about 30 μm, said metal powderbeing selected from the group consisting of electro-exploded aluminumpowder, electro-exploded titanium powder, electro-exploded copperpowder, electro-exploded zinc powder, and electro-exploded yttriumpowder, wherein said ignition material does not thermally decompose attemperatures up to about 120° C. and deflagrates when the heatingelement is heated to a temperature of at least about 250° C., whereinsaid ignition material is free of a binder and the mixture of metalpowder and particulate oxidizer being formed without the use of asolvent.
 15. The electrically actuatable igniter of claim 33 wherein theoxidizer is selected from the group consisting of alkali metal nitrates,alkaline earth metal nitrates, alkali metal perchlorates, alkaline earthmetal perchlorates, alkali metal chlorates, alkaline earth metalchlorates, ammonium perchlorates, ammonium nitrate, and mixturesthereof.
 16. The electrically actuatable igniter of claim 33 wherein theelectro-exploded metal powder is electro-exploded aluminum.
 17. Theelectrically actuatable igniter of claim 33 wherein the electro-explodedmetal powder is about 15% to about 75% by weight of the ignitionmaterial.
 18. The electrically actuatable igniter of claim 33 whereinthe amount of oxidizer is about 25% to about 85% by weight of theignition material.
 19. The electrically actuatable igniter of claim 33wherein the ignition material upon ignition produces an ignition productwith a temperature of about 3000° C. to about 6000° C.
 20. Theelectrically actuatable igniter of claim 33 wherein said metal powderhas a surface area of about 15 square meters per gram.
 21. Anelectrically actuatable igniter comprising: a pair of electrodes; aheating element electrically connected between said electrodes; and anignition material in contact with said heating element, said ignitionmaterial consisting essentially of a uniformly dispersed mixture of ametal powder and a particulate inorganic salt oxidizer, the metal powerbeing present in the ignition material in an amount of about 25% toabout 50%, by weight of the ignition material, said particulate oxidizerreacting exothermically with said metal powder, said particulateoxidizer having an average particle size of about 1 μm to about 30 μm,wherein said metal powder consists of electro-exploded aluminum powderand said ignition material deflagrates when the heating element isheated to a temperature of at least about 250° C., the mixture of metalpowder and particulate oxidizer being formed without the use of asolvent.
 22. The electrically actuatable igniter of claim 42 whereinsaid oxidizer is selected from the group consisting of alkali metalnitrates, alkaline earth metal nitrates, alkali metal perchlorates,alkaline earth metal perchlorates, alkali metal chlorates, alkalineearth metal chlorates, ammonium perchlorate, ammonium nitrate, andmixtures thereof.
 23. The electrically actuatable igniter of claim 42wherein said ignition material upon deflagration produces an ignitionproduct with a temperature of about 3000° C. to about 6000° C.